Chainsaw 371xp 6402040. What do numbers mean?

I'm assuming you mean air filter? If so, here is a video on how to change the air filter on a Husqvarna chainsaw, and a Redmax chainsaw, all of them are pretty similar so hopefully this will help you out!

Replacing the Air Filter on Husqvarna Chainsaw

Replacing the Air Filter on RedMax Chainsaw

Machine number? Do you mean model number? The serial number is stamped under the saw in the front.

FOLLOW this steps and fix it. Merry Christmas

Spark Plug

Every chainsaw relies on a spark plug to ignite the air-fuel mixture for proper combustion. Over time the electrode on the spark plug will burn away and the engine does not start. If the engine does not start, remove and inspect the spark plug. If there is any doubt that the plug is working properly replace it.
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Carburetor

If the chainsaw won't start, the carburetor might be clogged or have bad fuel in the float bowl. If old fuel was left in the chainsaw for a long time some of the volatile ingredients may have evaporated, leaving a thicker, stickier product that is more like varnish or shellac. This sticky fuel can clog up the small jets and ports in the carburetor and it can be difficult to burn making it hard to start the engine. The only solution is to drain the old fuel from the float bowl and thoroughly clean the carburetor with carburetor cleaner. If that isn't effective, replace the entire carburetor.
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On - Off Switch

Chainsaws are equipped with an on/off switch. If the chainsaw does not start, this switch might be defective. Check the switch for continuity. In the on position the switch should have continuity.
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Safety Switch

If the chainsaw does no start, the safety switch - or throttle control switch - might be defective. If the switch isn't engaging with the throttle it won't provide enough fuel to the engine and the engine does not start.
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Ignition Coil

If the chainsaw engine does not start the spark plug may not be getting voltage. Power to the spark plug is provided by the ignition coil. The ignition coil is mounted next to the flywheel. As the flywheel spins an electrical current is induced by the ignition coil. If the ignition coil is defective it will need to be replaced, it is not serviceable.
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Rewind Spring

The recoil starter assembly has a rewind spring which spins the pulley backward to rewind the starter rope. If the rewind assembly is broken the rope will not rewind and the engine does not start.
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Recoil Starter

If pulling the recoil starter assembly handle doesn't engage with the engine and the engine does not start. Repair or replace the recoil starter assembly.
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Recoil Starter Pulley

The recoil starter pulley winds up the starter rope when not in use. If the recoil starter pulley is broken or stuck it may not rewind the rope and the engine does not start.

test the steps and fix it. God bless you

Spark Arrester

Inside the muffler of most chainsaws is a screen-like device called a spark arrestor. If the chainsaw stops after a few seconds or won't start at all, the spark arrestor might be clogged. The spark arrestor prevents small embers from being emitted by the engine to prevent fires. Over time the spark arrestor will become clogged with soot. Remove the spark arrestor and clean with a wire brush or replace.
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Carburetor

If the chainsaw engine stops after a few seconds, the carburetor might be clogged or have bad fuel in the float bowl. If old fuel was left in the chainsaw for a long time some of the volatile ingredients may have evaporated, leaving a thicker, stickier product that is more like varnish or shellac. This sticky fuel can clog up the small jets and ports in the carburetor and it can be difficult to burn allowing the engine to only run for a few seconds. The only solution is to drain the old fuel from the float bowl and thoroughly clean the carburetor with carburetor cleaner. If that isn't effective, replace the entire carburetor.
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Gas Cap

The gas cap on YOUR chainsaw is designed to let a very small amount of air into the tank. This is necessary because as fuel is consumed a vacuum is created in the fuel tank. The gas cap has a tiny vent to let air into the tank and equalize the pressure. If the gas cap vent gets clogged air can't enter the tank and eventually the vacuum pressure in the tank exceeds the vacuum pressure of the engine, and the engine stalls. The vent often cannot be cleaned because it is made of a semi porous ceramic material. If the gas cap vent is clogged, replace the cap.
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Spark Plug

If the chainsaw starts and stops after a few seconds The spark plug may be defective. Although this is not very common it is easy to test. Try removing the spark plug and installing a new one.
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Fuel Filter

If the chainsaw stops after a few seconds, the fuel filter might be clogged. If old fuel was left in the chainsaw for a long time some of the volatile ingredients may have evaporated, leaving a thicker, stickier product that is more like varnish or shellac. This sticky fuel can clog up the fuel filter and it can be difficult to burn allowing the engine to only run for a few seconds. The only solution is to drain the old fuel from the fuel tank and replace the fuel filter.

Not all chainsaws are the same size, the numbers that you see on the kits are the size of the blades on the saw. You have to make sure that the sharpener you get is the right size for your saw.

Generally
H screw 1 full turn out anticlockwise
L screw 3/4 turn out anticlockwise
However some walbro carbs are 2 1/4 turns out on both H&L

Sounds like the chainbrake has activated, grab the hand guard and pull it firmly fully back to the top handle, it should click when it has re set.

Have you changed the ignition unit? Have you removed the starter cover and checked the Woodruf key for slippage?
Lou

there is a special tool to seperate the carnk case halfs.looks like a "c" clamp.they are very expensive to buy.there is no other way to seperate the case halfs with out distroying them......

Have your UT# available.

http://odref.com/homelite/chainsaw/index.html